wayno,
Kip Motors manufactures Metropolitan vent weatherstrip (4 37H9516 Weatherstrip, vent (pair) $175.45 at http://www.kipmotor.com/Catalogs/MET-glass.pdf).
I would anticipate that if the size of the 320 vent windows is similar to the 520, the cost would be similar.
You are free to call Kip Motors and inquire about a volume discount; however I would expect that you would have to prepay the entire order before he would begin volume production.
Please note, Kip is a businessman with integrity. He is very active in British car preservation, coordinates the All British & European Car Day (http://www.allbritishcarday.com/) and has been featured in numerous publications.
Also note, other than the assistance Kip has provided me in my Datsun projects (far removed from his typical British car customers) I have had no financial relationship with the business.
Unfortunately, I have no time to coordinate the production of the 520 vent seal, other than to loan an original set.
Keith